U.S. Pat. No. 4,871,506 explains that, in the case of semicrystalline fluid polymers, this tube blow extrusion process is at the very least tricky, if not impossible to use. In this case, the forces involved, such as the pulling force, the internal pressure in the bubble, gravity and other forces are applied to an extrudate whose mechanical strength is very low. This results in significant deformation of the bubble, leading to the formation of wrinkles in the film after the bubble has been laid flat, and to the complete absence of control of the distribution of film thickness.
The technique described in this patent U.S. Pat. No. 4,871,506 consists, using the blow coextrusion process, in coextruding the semicrystalline fluid polymer with a thermoplastic resin which is incompatible, in such a way that, after the bubble has been cooled and laid flat, the two extruded films are recovered separately by conventional means, such as by separate winding of the separated films. As examples of semicrystalline polymers, mention may be made of polyamides and their copolymers, polyesters such as polybutylene terephthalate (PBT) or polyethylene terephthalate (PET) and their copolymers, polypropylene, polyvinylidene fluoride and its copolymers, and ethylene-hydroxyalkyl ester copolymers (EVOH). Among incompatible thermoplastic resins that can be used, mention may be made of high-pressure polyethylenes, impact polystyrene and plasticized polyvinyl chloride.
The single example shows a bubble consisting of a 25 μm film of PVDF coextruded with a 60 μfilm of polyethylene. In the description, it is mentioned that the thickness of the film of incompatible thermoplastic resin (polyethylene) must preferably be from 1 to 5 times the thickness of the film of the semicrystalline polymer. It is also stated in the description that the coextrusion of more than two films is not excluded, but it is not known what films these are. In addition, in this technique the polyethylene film cannot be used as a film in itself and it must be recycled because it does not contain the necessary additives, such as antioxidants and UV stabilizers. If these additives are added thereto, the process, whose essential purpose is to produce a film of the semicrystalline polymer, is complicated. It is therefore desirable to reduce this thickness of the polyethylene film in order to reduce the amount that has to be recycled.
It has now been found that it is possible to produce, using the tube blow extrusion process, a film comprising a layer of fluoropolymer and a layer of another polymer (A), these two layers being adjacent, and a third layer of an incompatible polymer placed either on the same side as the fluoropolymer or on the same side as the polymer (A). The polymer (A) may be any polymer, for example an amorphous polymer, that is to say it is not necessary semicrystalline, and the thickness of the layer of incompatible polymer is not necessarily from one to five times the thickness of the combination formed by the layer of the fluoropolymer and the adjacent layer of the polymer (A). The layer of incompatible polymer is then separated.
It has also been found that it is possible to produce, using the tube blow extrusion process, a film comprising at least one layer of fluoropolymer containing fillers or at least one layer of polymer (A) and a layer of incompatible polymer. The layer of incompatible polymer is then separated.